A plunger lift is an apparatus that is used to increase the productivity of oil and gas wells. In the early stages of a well's life, liquid loading is usually not a problem. When rates are high, the well liquids are carried out of the well tubing by the high velocity gas. As a well declines, a critical velocity is reached below which the heavier liquids do not make it to the surface and start to fall back to the bottom exerting back pressure on the formation, thus loading up the well. A plunger system is a method of unloading gas in high ratio oil wells without interrupting production. In operation, the plunger travels to the bottom of the well where the loading fluid is picked up by the plunger and is brought to the surface removing all liquids in the tubing. The plunger also keeps the tubing free of paraffin, salt or scale build-up.
A plunger lift system works by cycling a well open and closed. During the open time, a plunger interfaces between a liquid slug and gas. The gas below the plunger will push the plunger and liquid to the surface. This removal of the liquid from the tubing bore allows an additional volume of gas to flow from a producing well. A plunger lift requires sufficient gas presence within the well to be functional in driving the system. Oil wells making no gas are thus not plunger lift candidates.
As the flow rate and pressures decline in a well, lifting efficiency declines geometrically. Before long the well begins to “load up”. This is a condition whereby the gas being produced by the formation can no longer carry the liquid being produced to the surface. There are two reasons this occurs. First, as liquid comes in contact with the wall of the production string of tubing, friction occurs. The velocity of the liquid is slowed, and some of the liquid adheres to the tubing wall, creating a film of liquid on the tubing wall. This liquid does not reach the surface. Secondly, as the flow velocity continues to slow, the gas phase can no longer support liquid in either slug form or droplet form. This liquid along with the liquid film on the sides of the tubing begin to fall back to the bottom of the well. In a very aggravated situation, there will be liquid in the bottom of the well with only a small amount of gas being produced at the surface. The produced gas must bubble through the liquid at the bottom of the well and then flow to the surface. Because of the low velocity, very little liquid, if any, is carried to the surface by the gas. Thus, a plunger lift will act to remove the accumulated liquid.
A typical installation plunger lift system 100 can be seen in FIG. 1. Lubricator assembly 10 is one of the most important components of plunger system 100. Lubricator assembly 10 includes cap 1, integral top bumper spring 2, striking pad 3, and extracting rod 4. Extracting rod 4 may or may not be employed depending on the plunger type. Contained within lubricator assembly 10 is plunger auto catching device 5 and plunger sensing device 6.
Sensing device 6 sends a signal to surface controller 15 upon plunger 200 arrival at the well top. Plunger 200 can represent the plunger of the present invention or other prior art plungers. Sensing the plunger is used as a programming input to achieve the desired well production, flow times and wellhead operating pressures.
Master valve 7 should be sized correctly for the tubing 9 and plunger 200. An incorrectly sized master valve 7 will not allow plunger 200 to pass through. Master valve 7 should incorporate a full bore opening equal to the tubing 9 size. An oversized valve will allow gas to bypass the plunger causing it to stall in the valve.
If the plunger is to be used in a well with relatively high formation pressures, care must be taken to balance tubing 9 size with the casing 8 size. The bottom of a well is typically equipped with a seating nipple/tubing stop 12. Spring standing valve/bottom hole bumper assembly 11 is located near the tubing bottom. The bumper spring is located above the standing valve and can be manufactured as an integral part of the standing valve or as a separate component of the plunger system. Fluid accumulating on top of plunger 200 may be carried to the well top by plunger 200.
Surface control equipment usually consists of motor valve(s) 14, sensors 6, pressure recorders 16, etc., and an electronic controller 15 which opens and closes the well at the surface. Well flow ‘F’ proceeds downstream when surface controller 15 opens well head flow valves. Controllers operate on time, or pressure, to open or close the surface valves based on operator-determined requirements for production. Modern electronic controllers incorporate features that are user friendly, easy to program, addressing the shortcomings of mechanical controllers and early electronic controllers. Additional features include: battery life extension through solar panel recharging, computer memory program retention in the event of battery failure and built-in lightning protection. For complex operating conditions, controllers can be purchased that have multiple valve capability to fully automate the production process.
Modern plungers are designed with various sidewall geometries and can be generally described as follows:    A. Shifting ring plungers for continuous contact against the tubing to produce an effective seal with wiping action to ensure that all scale, salt or paraffin is removed from the tubing wall. Some designs have by-pass valves to permit fluid to flow through during the return trip to the bumper spring with the by-pass shutting when the plunger reaches the bottom. The by-pass feature optimizes plunger travel time in high liquid wells.    B. Pad plungers have spring-loaded interlocking pads in one or more sections. The pads expand and contract to compensate for any irregularities in the tubing, thus creating a tight friction seal. Pad plungers can also have a by-pass valve as described above.    C. Brush plungers incorporate a spiral-wound, flexible nylon brush section to create a seal and allow the plunger to travel despite the presence of sand, coal fines, tubing irregularities, etc. By-pass valves may also be incorporated.    D. Solid plungers have solid sidewall rings for durability. Solid sidewall rings can be made of various materials such as steel, poly materials, Teflon®, stainless steel, etc. Once again, by-pass valves can be incorporated.    E. Snake plungers are flexible for coiled tubing and directional holes, and can be used as well in straight standard tubing.
FIGS. 2A, 2B, 2C and 2D are side views of various plunger mandrel embodiments. All geometries described have an internal orifice.    A. As shown in FIG. 2C, plunger mandrel 20 is shown with solid ring 22 sidewall geometry. Solid sidewall rings 22 can be made of various materials such as steel, poly materials, Teflon®, stainless steel, etc. Inner cut grooves 30 allow sidewall debris to accumulate when a plunger is rising or falling.    B. As shown in FIG. 2D, plunger mandrel 80 is shown with shifting ring 81 sidewall geometry. Shifting rings 81 allow for continuous contact against the tubing to produce an effective seal with wiping action to ensure that all scale, salt or paraffin is removed from the tubing wall. Shifting rings 81 are individually separated at each upper surface and lower surface by air gap 82.    C. As shown in FIG. 2A, plunger 60 has spring-loaded interlocking pads 61 in one or more sections. Interlocking pads 61 expand and contract to compensate for any irregularities in the tubing, thus creating a tight friction seal.    D. As shown in FIG. 2B, plunger 70 incorporates a spiral-wound, flexible nylon brush 71 surface to create a seal and allow the plunger to travel despite the presence of sand, coal fines, tubing irregularities, etc.
Recent practices toward slim-hole wells that utilize coiled tubing also lend themselves to plunger systems. Because of the small tubing diameters, a relatively small amount of liquid may cause a well to load-up, or a relatively small amount of paraffin may plug the tubing.
Plungers use the volume of gas stored in the casing and the formation during the shut-in time to push the liquid load and plunger to the surface. This plunger lift occurs when the motor valve opens the well to the sales line or to the atmosphere. To operate a plunger installation, only the pressure and gas volume in the tubing/casing annulus is usually considered as the source of energy for bringing the liquid load and plunger to the surface.
The major forces acting on the cross-sectional area of the bottom of the plunger are:                The pressure of the gas in the casing pushes up on the liquid load and the plunger.        The sales line operating pressure and atmospheric pressure push down on the plunger.        The weight of the liquid and the plunger weight itself push down on the plunger.        Once the plunger begins moving to the surface, friction between the tubing and the liquid load acts to oppose the plunger.        In addition, friction between the gas and tubing acts to slow the expansion of the gas.        
In certain wells, the well bottom consists of sand. FIG. 1A (prior art) is a blow up schematic of a well bottom section 600 showing accumulated water 17 and sand 13 trapped within inner cut grooves 30. Sand 13 tends to cake up within the inner cut grooves 30 and on the sidewall rings 22 of the plunger which will hinder the plunger operation. Solid ring plungers tend to get sand between each sidewall ring 22. Shifting ring, pad or brush plungers can also tend to cake with sand. When plungers are caked with sand, they tend to get caught within the aforementioned lubricator and may require manual intervention (maintenance). In addition, a major disadvantage of using prior art plunger lifts in a sandy well is that these plungers will cake with sand and fail to fall, or fall too slowly, to the bottom of the well. When plunger drop travel time slows, well production can be limited. Also, fishing a plunger out of a well can be a problem and may sometimes require the pulling of the complete tubing string.
What is needed is a plunger lift apparatus that is capable of being used in a sand-bottom well and which cleans itself. A clean plunger results in continuous efficiency. It drops back to the well bottom quickly and easily, where it can assist in increasing lift cycle times, thereby optimizing well production. What is also needed is a self-cleaning plunger system for sandy wells that may be retrievable from the well. The apparatus of the present invention provides a solution to these aforementioned issues.